This invention pertains to lifting apparatus having individual hydraulic cylinders in series for increasing upward extension. Particularly, a first parallel arrangement of cylinders is operative to lift an upper stage of an extensible stabilizing tower, and then another cylinder supported by the upper stage is operative to lift a load to a greater height.
Conventionally, a plurality of coaxial, telescoping hydraulic cylinders are used to provide in a single device greater extension than can be obtained by use of a simple lifting hydraulic cylinder having only one cylinder and one piston rod. A commercially available lifter utilizing coaxial cylinders is stabilized by a surrounding tower comprising a plurality of stages of closely fitting members with square or rectangular cross sections. When such a lifter has several cylinders, the outer cylinder is much larger than the inner cylinder and therefore has much greater capacity for lifting than is available at the higher levels where the cylinders are smaller. The complete assembly is big, heavy, and expensive.
In U.S. Pat. No. 4,336,840 issued to Thomas F. Bailey on Jun. 29, 1982, two cylinders operate in series in a structure that is not applicable to usual lifting jobs. The cylinders are connected together at the base ends such that the piston rods extend vertically in opposite directions. The bases are connected to a carrier that slides up and down a tower of fixed length, and the distal ends of the piston rods are connected respectively to the top and to the bottom of the tower.